1. Field of the Invention
The present invention relates generally to radio base station systems, synchronization window control methods and synchronization window control programs, and particularly to a radio base station system permitting path division multiple connection of a plurality of mobile terminal devices in a mobile communication system, and a synchronization window control method and a synchronization window control program for providing an appropriate timing of signal reception (a synchronization position) in such a radio base station system.
2. Description of the Background Art
In recent years, in rapidly developing mobile communication systems (for example the personal handy phone system (PHS)) a path division multiple access (PDMA) system has been proposed to enhance radio wave frequency utilization efficiency. In this system, a single time slot of a single frequency can spatially be divided to allow path division multiple connection of mobile terminal devices of a plurality of users to a radio base station system. In the system, a signal from each user's mobile terminal device is separated and extracted by a well-known adaptive array process.
In a mobile communication system according to such a PDMA system, a reception timing at which a signal transmitted from each mobile terminal device arrives at a radio base station (hereinafter also referred to as a “synchronization position”) varies for a variety of factors, such as a variation of the distance between the terminal device and the base station due to the movement of the terminal device, radio wave propagation characteristics variation, and the like.
In a mobile communication system of the PDMA system, when mobile terminal devices of a plurality of user are permitted to have path division multiple connection to a single time slot, the synchronization positions of signals received from mobile terminal devices, respectively, may vary for the aforementioned reasons and thus approach to each other or in some case have an inverted time relationship therebetween.
If synchronization positions approach each other too close, signals received from a plurality of mobile terminal devices would have a high temporal correlation. This results in low precision extraction of a signal for each user by the adaptive array process and consequently impaired speech characteristics for each user.
Furthermore in the PHS a signal received from each mobile terminal device includes a reference signal section for each frame. The reference signal section is formed of a known bit train shared by all users. If the synchronization positions of signals from the mobile terminal devices of multiple users match, the received signals would have their respective reference signal sections overlapping and the users cannot be identified or separated from each other, and inter-user interference (so-called “swap”) would be introduced.